The present invention relates generally to digital wireless communication, and more specifically to frequency modulation using a voltage-controlled oscillator (VCO).
The proliferation of digital wireless communication systems has led to a need for reliable modulation circuitry. Phase-locked loop (PLL) circuits are attractive in modulation applications due to their combination of controllable modulation and stable so and adjustable carrier frequency.
As known in the art, a VCO can be used to effect Gaussian-shaped Continuous Phase Binary Frequency Shift Keying Modulation (GCPBFSK) in which the modulating wave shifts the output frequencies, away from the center frequency, by an amount between predetermined values. The predetermined values, i.e., two different frequencies representing zero and one, are called modulation deviation and are denoted by f0 and f1 respectively. The frequencies, f0 and f1 are typically equal. For example, for a wireless standard having 79 channels in a frequency range of 2402 MHz-2480 MHz with each channel separated by 1 MHz, the center frequencies may be equal to 2402 MHz+nMHz (n=0 to 78). The f0 may range within some negative values (e.g., from xe2x88x92140 KHz to xe2x88x92175 Khz) and f1 may range within a corresponding positive values (e.g. from 140 KHz to 175 KHz), in respect to the center frequency.
In order to keep frequencies f0 and f1 within their specific range, the voltage to the VCO must be controlled very carefully. Typical VCO gain (KVCO) may be on the order of 110 MHz/V-200 MHz/V. Typical VCOs may be operated from 0.6 v to 2.2 v, but actually only the linear range (1.0 v-1.8 v) can be used for this application, otherwise non-linear distortion will result.
For a wide frequency range (2402 MHz-2480 MHz), it is difficult to maintain a linear frequency response. The oscillation frequency of a VCO circuit is inversely proportional to the square root of the product of the inductance and capacitance (LC). It is possible to divide a large frequency range into segments to obtain a piecewise linear response over each segment. This can be achieved by using a set of switch-able capacitors, varactors (whose capacitance is voltage-controllable) and inductors. Often a larger frequency range is divided to account for the tolerance values of integrated capacitors and inductors that may run as high as 30%. Each segment is then covered by a specific capacitor""s combination.
The voltage-frequency characteristics of a VCO vary from chip to chip. Therefore to obtain the appropriate modulation characteristics, the voltage to the VCO must be controlled depending on the characteristics of the chip. A typical PLL circuit in accordance with the prior art includes a charge pump, which acts as a current source. The charge pump consists of a set of current sources. The appropriate current source is chosen, based on the VCO voltage-frequency characteristics, to charge the capacitors of the LPF and thus controls the voltage to the VCO.
The difficulty arises in determining the VCO capacitance value (i.e. the capacitors"" combination) and charge pump current value to provide the desired modulation deviation for each channel.
A method for setting the VCO circuit capacitance value and charge pump current level is described. To set the capacitance value, the operation frequency range is divided into a plurality of frequency bands. Then the frequency of a voltage-controlled oscillator (VCO) for each combination of capacitance values and lowest/highest control voltage levels is measured. A capacitance value for each frequency band is then selected such that the frequency band is between the lowest frequency (corresponding to the lowest voltage level) and highest frequency (corresponding to the highest control voltage).
To set the charge pump current level, a representative frequency is determined for each frequency band. Then VCO gain at the representative frequency is estimated and the charge pump current level for each frequency band is selected based upon the estimated VCO gain.
Other features and advantages of the present invention will be apparent from the accompanying drawings, and from the detailed description, that follows below.